1. Field of the Invention:
This invention relates generally to magnetic tape monitoring apparatuses, and more particularly to an automatic character skew and spacing checking network for use with computer digitial magnetic tapes.
2. Description of the Prior Art:
In the recent past Government and Industry have been using computers with digital tape drives that permit high density recording, up to 800 characters per inch for example, recorded in the non-return to zero (NRZI) mode. As more equipment of this type is used, a substantial increase in the number of character reading failures has occurred. It has been found that the largest number of reading failures occurs when tapes are read by equipment different from that on which they were recorded, giving rise to serious compatibility problems. Reading failures have occurred in spite of the fact that equipment was maintained to the manufacturers specifications, and in spite of the fact that the most rigorous servicing standards were observed in adjusting and maintaining reading and recording equipment.
Extensive tests directed to resolving these problems have shown that the most common cause of reading failures when attempting to interchange information via digital tape recorded in the non-return to zero mode is a condition called skew. Skew is a product of two factors which result when the characters are not recorded perpendicular to the edge of the recording tape. The first of these factors is static skew, determined by the physical alignment of the recording head and tape guides and also by the alignment of the individual track recording gaps within the head assembly (i.e. gap scatter). The other factor is dynamic skew, a result of the wandering and squirming of the tape as it passes across the recording head. Other factors such as the asymmetry and pattern sensitivity of the recording head also contribute to the overall skew condition. All of these factors are generally lumped together and referred to as "skew" because in practice they are inseparable from a measurement point of view. It has been reliably determined that tape skew accounts for 95% of reading errors due to equipment incompatibility.
FIG. 1 illustrates characters recorded on magnetic rape, showing the effects of skew conditions. In particular, two characters 12 and 14 of recorded data bits 16 are shown at the left of FIG. 1, representing ideal recording conditions with no skew and with a fixed gap 18 between all bits in each of the characters. Two skewed characters 20 and 22 are illustrated at the right side of FIG. 1, showing the effects of gap scatter and the other static and dynamic skewing phenomena mentioned above.
Concerning the need for a sufficient gap between characters, it is emphasized that the reading problems discussed here relate to non-return to zero recording, rather than to phase encoded recording wherein the gap between characters is not critical since the recorded information is self-clocking. In non-return to zero recording, particularly with a packing density of 800 characters per inch, character spacing becomes critical as a character must be written every 1250 microinches with an accuracy of 3%. Since an adequate spacing must exist between characters to prevent ambiguous readouts, character skew must be less than half of the character spacing minus an additional tolerance to provide for read head skew which can never be entirely eliminated.
FIGS. 2A and 2B are diagrams showing gap relationships and illustrating how character reading errors can occur due to insufficient gapping as a result of skew. FIG. 2A illustrates a 9-track tape showing track 1 at the top and track 9 at the bottom of the tape. A first character of 9 skewed bits is illustrated at 24 and a second similarly skewed character is illustrated at 26, both characters separated by a minimum distance of 825 microinches. Both characters 24 and 26 are illustrated as skewed 425 microinches, the maximum writing skew permitted under the American National Standard for recording magnetic tape for information interchange (800 cpi, NRZI). In FIG. 2A the first bit occurs on track 9 on the tape. This bit will trigger a window pulse in the reading apparatus which must be open for at least 425 microinches in order to read a tape written at the maximum skew tolerance mentioned above. The second skewed character 26 is similar to the first in that the first bit also occurs in track 9, so that the recording system leaves a very adequate spacing of 825 microinches between the closest bits of the two skewed characters. Accordingly no reading ambiguity occurs in this situation.
FIG. 2B illustrates the critical worst-case problem where reading ambiguity may occur. A character with only 1 bit in track 1 is illustrated at 28 and a second character with only 1 bit in track 9 is illustrated at 30. Thus, the reading window is opened only as the bit 28 is detected, so that this track 1 bit falls at the beginning of the reading window rather than at the end of it, in contrast to the situation shown in FIG. 2A. The next recorded character 30 has at least one bit in track 9 which, in the illustrated example, is skewed most forward. Thus this bit triggers a second reading window which begins only 400 microinches after the end of the first reading window. Within this 400 microinch gap, a 6% character tolerance must also be provided, equivalent to subtracting 75 microinches from the actual gap. Thus the spacing between the characters is effectively only 325 microinches. Therefore, to maintain reliable operation in the illustrated example, read head static and dynamic skew must be less than 300 microinches, significantly less than the 425 microinch tolerance conventionally permitted.
The 6% tolerance mentioned in the previous paragraph is included since a maximum of .+-. 3% in character spacing tolerance is permitted by recording specifications and a .+-. 3% speed tolerance is the usual specified by the tape drive manufacturers.
Speed variations that reduce the character spacing may also impair the tape drive's ability to distinguish between characters. For example, with a recording density of 800 cpi, a character is written every 11.8 microseconds. If the tape is moving slower than the normal speed of 112.5 inches per second, the characters will be spaced closer together with the result that the characters may be spaced too closely to be read by other tape drives, or even by the writing tape drive when it is not stopped between records.
In view of the problems described above, it is apparent that magnetic tape recording equipment must be carefully monitored to minimize the skew in recording and reading heads if recorded tapes are to be compatable with different types and models of equipment. No satisfactory equipment has been available in the past for providing such monitoring in a truly convenient and reliable manner.
A need therefore exists for an apparatus to automatically check character skew and spacing to ensure that recorded tapes will be compatible with different types of reading equipment.